The Hebrew terms for "oath," "alah" and "shebu'ah," are used interchangeably in the Old Testament (comp. Gen. xxiv. 8 and 41; see also Shebu. 36a). According to the ancient Jewish principle of jurisprudence the judicial oath was employed in civil cases only, never in criminal cases, and only in litigations concerning private property, never in those over sacerdotal property; and over movable but not over immovable property. Later rabbinical law, however, impose⋅ the oath even in the case of sacerdotal and immovable property (Shebu. 42b; Shulḥan 'Aruk, Ḥoshen Mishpaṭ, 95, 1). Moreover, the oath was administered only in case no evidence, or only insufficient evidence, was forthcoming on either side (Shebu. 45a, 48b). But if there was ample evidence, documentary or oral, i.e., that of witnesses (Weiss, "Dor," i. 202), in support of the statements of the litigants, or if claim and disclaimer were not positive, no oath could be imposed, according to Biblical law. The character of the oath was, then, in the nature of a rebuttal of the claim of the plaintiff and was imposed upon the defendant as a purgative measure; or God was called upon as a witness, there being no other. Adelung, indeed (in Saalschütz, "Archäologie," p. 277), derives the German "eid" (oath) from the Hebrew "'ed" (="witness"); see also Delitzsch (on Gen. xxi. 30), Trumbull ("Blood Covenant," p. 266), Tyler ("Oaths," p. 87), and Paley ("Principles of Moral and Political Philosophy," book iii., ch. 16, p. 1).

Though the oath was not considered as full legal evidence (Frankel, "Der Gerichtliche Beweis," p. 305), it was accepted in lieu of something better. However, the oath was not meant to be an ordeal, a means of frightening the contestant into telling the truth, except in the case of the oath of purgation administered to a woman suspected of adultery (Num. v. 21; Michaelis, "Das Mosaische Recht," section 301, p. 341). The perjurer, however, was not liable to the court, but to God Himself (see Perjury); furthermore, to such as were suspected of a disposition not to speak the truth the oath was not administered (Shebu. vii. 1). The Biblical oath was imposed only upon the defendant (Shebu. vii. 1; Ḥoshen Mishpaṭ, 89, 1; "Yad," Ṭo'en, i. 2). The reason for this is in the dictum, "Possession is nine-tenths of the law"; or, as the Talmud (B. Ḳ. 46a) states it, "Whosoever would oust a possessor must bring evidence to establish his claim; his positive assertion alone is not sufficient, for the possessor may take the oath in support of his equally positive denial of plaintiff's claim."

The codifiers classify the Biblical oaths under the following three divisions: (1) "Oaths of keepers or custodians": A leaves certain objects in the care of B; B admits having received them, but claims that they have been stolen or lost; he takes the oath in support of his assertion and is acquitted from responsibility (Shebu. viii. 1; B. M. 93a; B. Ḳ. 107b; "Yad," She'elah, iv. 1, vi. 3; Ḥoshen Mishpaṭ, 87, 7). (2) "Part admittance": A claims to have lent B 100 shekels; B admits the claim as regards only 50, and after taking the oath is acquitted; but if B repudiates the claim in its entirety he is acquitted without oath. (3) But if A has one witness in proof of his claim, B must take the oath in either case (Shebu. vi. 1; B. M. 3a; Ḥoshen Mishpaṭ, l.c.). The admissions of B in cases 1 and 2, and the statement of the one witness in case 3, are considered as half-evidence (Frankel, l.c.) in support of A, but not as sufficient to warrant a judgment in his favor. B, therefore, takes the oath, which is equal to half-evidence, and thereby invalidates the claim of A.

Though Biblical legislation imposed the oath only upon the defendant, changed times and conditions rendered it necessary for the teachers of Mishnah to impose the oath at times even upon the plaintiff, in cases where the defendant is not competent to take the oath, or where the claim of plaintiff has evidently a greater probability of truth than the disclaimer of defendant. These oaths are known as "mishnaic" oaths; and while the Biblical oaths are of a purgative nature, the mishnaic oaths are "oaths of satisfaction" ("nishba'in we-noṭelin"—"they swear, and their claim is satisfied"; Shebu. vii. 1). The following cases fall within this category: (1) The defendant should take the Biblical oath, but he is suspected of a disposition to swear falsely; the court can not, therefore, administer the oath to him, and imposes the mishnaic oath upon the plaintiff instead. (2) It is imposed further in the case of a laborer claiming wages; (3) of a storekeeper claiming settlement for goods ordered; (4) of one who claims compensation for robbery; and (5) of one who claims compensation for battery, certain indications supporting the claim. In all these cases, the claims being based upon positive charges, while the disclaimer is not quite positive, the plaintiff takes the oath and secures judgment (Shebu. vii. 1; Ḥoshen Mishpaṭ, 89, 1; 90, 1; 91, 1; 92, 1). In some cases the mishnaic oath is imposed when the claim is not positive, as in the cases of partners, renters (paying part of the crop for rent), guardians (appointed by the court), and stewards. All these must take the mishnaic oathof purgation if accused of unscrupulous conduct, even though the claim is based only on a vague suspicion (Shebu. 45a, 48b; "Yad," Shutfin, ix. 1; Ḥoshen Mishpaṭ, 93, 1).

Later Talmudical practise has imposed the oath in cases where according to Biblical and mishnaic law no oath was imposed, as, for instance, where a claim is repudiated in its entirety. The originator of this form of oath, known as the "imposed oath" (; see Rashi to B. M. 5a), was the Babylonian Naḥman ben Jacob (235-324 C.E.; Shebu. 40b; B. M. 5a; Ḥoshen Mishpaṭ, 87, 7).

There is still another form of oath—the "oath of adherence" ("gilgul shebu'ah"; see Frankel, l.c.; Mendelssohn, however ["Jahrb. für Preussische Gesetzgebung," cxvi. 414], calls it "Neben-eid"). If one of the litigants is compelled to take the oath and his opponent seizes the opportunity to confront him with a second claim, upon which, had it been made separately, no oath would have been taken, the second claim is "adhered" to the first claim, and the defendant must take the oath in connection with both claims (Ḳid. 28a; Shebu. 48b). The only exception to this rule is made in favor of the laborer claiming his wages. To his oath no other claim can be attached: "it should rather be made as easy as possible for him" (Ḥoshen Mishpaṭ, 89, 6). In any other case there is no difference; whatever the oath, a second claim may always be "adhered" to it (Shebu. 48b; Ḥoshen Mishpaṭ, 94, 1). In other respects, however, there are far-reaching differences among the Biblical, the mishnaic, and the rabbinical oaths, both in practise and in principle.

(1) If the Biblical oath is required and the defendant will not take it, judgment is rendered against him, and his property is levied upon; if the mishnaic oath is required and the defendant will not take it, judgment is rendered against him, but his property can not be attached, and only a thirty days' ban is issued against him; or, if this be of no avail, slight corporal punishment is inflicted upon him by the court; but if it is the rabbinical oath that he refuses to take, not even judgment can be rendered against him; instead he is acquitted (Shebu. 41a; Ḥoshen Mishpaṭ, 87, 9; B. B. 33a).

(2) If the Biblical oath is required but it can not be administered to the defendant on account of his immoral character or because it is suspected that he would swear falsely, the plaintiff takes the oath and secures judgment; if the mishnaic oath is required but the defendant is not admitted thereto for the reasons stated, he is acquitted without oath ("Yad," Ṭo'en, iii. 4; Ḥoshen Mishpaṭ, 87, 13). He must, however, according to later rabbinical practise, take the rabbinical oath (B. B. 33a; Ḥoshen Mishpaṭ, 92, 9), if the claim against him is based upon a positive statement; but if it is the rabbinical oath that is required and he is not permitted to take it for the reasons given, the defendant is acquitted without any oath (Ḥoshen Mishpaṭ, 92, 10).

(3) The Biblical or mishnaic oath, whether of satisfaction or of purgation, when required of the defendant can not be imposed instead upon the plaintiff if he is not willing to take it; but the rabbinical oath may be so imposed (Shebu. 41a; Ḥoshen Mishpaṭ, 87, 11).

(4) The Biblical oath is imposed only if claim and disclaimer are positive; the mishnaic or the rabbinical oath, even if they be vague and uncertain (Shebu. vii. 1).

(5) The Biblical oath is imposed only when the object in litigation is private and movable property, and not if it is sacerdotal or immovable property; the rabbinical oath is imposed even in cases involving sacerdotal or immovable property (Shebu. vii. 1; Ḥoshen Mishpaṭ, 95, 1).

(6) If the Biblical or the mishnaic oath is imposed, the juror must swear by the name of Yhwh and must hold a Bible or a sacred object in his hands, and the judge must admonish the juror and impress upon him the sacredness and the importance of the oath. The judge must also warn him against any mental reservation or ambiguity; but if he takes the rabbinical oath he may not mention the name of Yhwh: he says merely, "I swear—." He need not hold in his hands a sacred object, and it is not necessary for the judge to admonish or warn him (Shebu. 36a; Ḥoshen Mishpaṭ, 87, 13, 20, 21; see Frankel, "Die Eidesleistung," p. 31; Mendelssohn, in "Jahrb. für Preussische Gesetzgebung," cxvi. 414, in reference to gradation of oath).

The Geonim have extended the oath even to cases where the Talmud does not impose it, as when minors are concerned, if the oath is in their favor (Ḥoshen Mishpaṭ, 89, 2); and sometimes they have put even witnesses under oath, though legally the latter are not called upon to swear (Ḥoshen Mishpaṭ, 28, 2; Frankel, l.c. p. 212). The Bible, however, mentions only the following cases in which a judicial oath was required: a keeper suspected of careless watching or of taking a piece of property entrusted to him for safe-keeping (Ex. xxii. 7-10), and a woman suspected by her husband of adultery (Num. v. 22); and enlarged and amplified as was the scope of the oath in post-Biblical times, it was still restricted to civil courts. In criminal cases no oath was employed, as, according to Jewish principles of jurisprudence, no one charged with a criminal act could be believed even upon oath. Assault and battery and embezzlement were considered only from their civil side, in regard to liability for damages. Nor was a witness, even in civil matters, put under oath, for "if we can not believe him without an oath we can not believe him at all" (Tos. Ḳid. 43b). The adjuration of a witness mentioned in Lev. v. 1 refers to a private adjuration for one to appear and testify as to what he knows about the case, but not to judicial adjuration (see commentaries of Keil and Delitzsch ad loc.; comp. Shebu. 35a).

The court may adjure the witness if it sees fit, and such, indeed, was the practise in geonic times, but it is not obliged to do so (Frankel, l.c. p. 212), as the character of the witness is assumed to be one of probity and above suspicion or reproach (B. Ḳ. 72b; Sanh. 25a; Ḥoshen Mishpaṭ, 34, 1; Josephus, "Ant." iv. 8, § 15). In short, the oath was not the only means by whicha statement could be supported; on the contrary, it was employed only for want of better evidence. But if there were witnesses, their statements were accepted as full legal evidence, and they were not subjected to an oath; for to swear falsely is not more sinful than to utter an ordinary lie. It is one's duty to speak the truth whether without or with an oath.

Neither were there any oaths for rulers or for subjects or citizens as such. The "oath of fidelity" that Herod required from the people ("Ant." xv. 10, § 4) was adopted from Roman custom. Nor were there priestly oaths. Yoma i. 5 refers not to an oath taken upon entering office, but to the oath of service, introduced as a check upon Sadduceeism. There were no official oaths of any kind, for officials were supposed to do their duty without the oath. "The multiplication of oaths," says Michaelis (l.c. section 301, p. 342), "tends rather to the corruption of morals." Paley (l.c. p. 144) also deplores the fact that "a pound of tea can not travel regularly from the ship to the consumer without costing at the least a half-dozen of oaths." The Jewish law, then, knew only judicial-oaths.

But while legally recognized oaths were limited to judicial proceedings, extrajudicial oaths were employed freely in private life. The nature of the oaths mentioned in Num. xxx. 2 places them in the category of vows; they were employed merely "as props to a weak will," and "were taken in order the better to uphold the Law" (Ḥag. 10a). These extrajudicial oaths were: oaths of agreement—e.g., between Abraham and Abimelech (Gen. xxi. 23), Isaac and Abimelech (ib. xxvi. 31), Jacob and Esau (ib. xxv. 33), Jacob and Laban (ib. xxxi. 53), Joshua and the Gibeonites (Josh. ix. 16), Zedekiah and Nebuchadnezzar (II Kings xxiv. 20; II Chron. xxxvi. 13); oaths of promise—e.g., between Abraham and Melchizedek (Gen. xiv. 22), Abraham and Eliezer (ib. xxiv. 3), Jacob and Joseph (ib. xlvii. 31), Joseph and his brothers (ib. 1. 25), Rahab and the spies (Josh. ii. 12), David and Jonathan (I Sam. xx. 3, 13), Saul and the woman of Endor (ib. xxviii. 10), David and Shimei (II Sam. xix. 23); oaths of adjuration—e.g., those of Deut. xxvii. 15, Josh. vi. 26, and I Sam. xiv. 24. Though strictly speaking these were not oaths in a judicial sense, they were, nevertheless, recognized as morally binding and as necessary to national security (II Sam. v. 3; Esth. x. 5; Josephus, "Ant." xv. 10, § 4), and even to international security (II Chron. xxxvi. 13; II Kings xxiv. 20; see Ned. 65a and Manasseh ben Israel in Mendelssohn's "Werke," iii. 248). Even if fraudulently obtained (Josh. ix. 16) or erroneously made (I Sam. xiv. 24; Judges xi. 35; Josephus, "Vita," § 53) the oath was considered inviolable. Even the silent determination of the heart was considered as the spoken word which must not be changed (Mak. 24a; B. M. 44a). The general principle was, "Let thy 'yea' be 'yea' and thy 'nay,' 'nay'" (B. M. 49a; comp. Matt. v. 37; James v. 12). The Law had already placed a careful restriction upon the practise of oath-taking in the case of a member of a family other than the head (Num. xxx.), and in post-exilic times people seem indeed to have been more careful in regard to taking oaths (Eccl. ix. 2); the prophet Zephaniah conceived the possibility of avoiding the oath altogether (iii. 13). The Essenes also avoided swearing, which they esteemed worse than perjury; "he that can not be believed without an oath is already condemned" (Josephus, "B. J." ii. 8, § 6). Philo says, "The bare word of a virtuous man should be like an oath, steadfast, inviolable, and true. Should necessity absolutely require an oath let a man swear by his father and mother . . . instead of by the name of the highest and first Essence." Even in judicial oaths, swearing by the name of Yhwh was abolished altogether during the age of the Geonim (Rashi to Shebu. 38b; Ḥoshen Mishpaṭ, 87, 19). See Covenant; Evidence; Perjury; Vows.